Conventional low density polyethylene has been historically polymerized in heavy walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures up to 300.degree. C. The molecular structure of high pressure, low density polyethylene (HP-LDPE) is highly complex. The permutations in the arrangement of their simple building blocks are essentially infinite. HP-LDPE's are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of these resins. HP-LDPE's also possess a spectrum of short chain branches, generally 1 to 6 carbon atoms in length. These short chain branches disrupt crystal formation and depress resin density.
With recent developments in low pressure technology, low density polyethylene can now be produced at low pressures and temperatures by copolymerizing ethylene with various alphaolefins. These low pressure LDPE (LP-LDPE) resins generally possess little, if any, long chain branching. They are short chain branched with branch length and frequency controlled by the type and amount of comonomer used during polymerization.
U.S. patent application Ser. No. 892,325 filed Mar. 31, 1978, now abandoned, and refiled as Ser. No. 014,414 on Feb. 27, 1979, now U.S. Pat. No. 4,302,566, in the names of F. J. Karol et al. and entitled Preparation of Ethylene Copolymers In Fluid Bed Reactor, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities if the monomer(s) are copolymerized in a gas phase process with a specific high activity Mg-Ti containing complex catalyst which is blended with an inert carrier material.
U.S. patent application Ser. No. 892,322 filed Mar. 31, 1978, now abandoned, and refiled as Ser. No. 012,720 on Feb. 16, 1979, now U.S. Pat. No. 4,302,565, in the names of G. L. Goeke et al. and entitled Impregnated Polymerization Catalyst, Process For Preparing, and Use For Ethylene Copolymerization discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow rate of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities, if the monomer(s) are copolymerized in a gas phase process with a specific high-activity Mg-Ti-containing complex catalyst which is impregnated in a porous inert carrier material.
U.S. patent application Ser. No. 892,037 filed Mar. 31, 1978, now abandoned, and refiled as Ser. No. 014,412 on Feb. 27, 1979, in the names of B. E. Wagner et al. and entitled Polymerization Catalyst, Process For Preparing And Use For Ethylene Homopolymerization, discloses that ethylene homopolymers having a density of about .gtoreq.0.958 to .ltoreq.0.972 and a melt flow ratio of about .gtoreq.22 to about .ltoreq.32 which have a relatively low residual catalyst residue can be produced at relatively high productivities for commercial purposes by a low pressure gas phase process if the ethylene is homopolymerized in the presence of a high-activity Mg-Ti-containing complex catalyst which is blended with an inert carrier material. The granular polymers thus produced are useful for a variety of end-use applications.
The polymers as produced, for example, by the processes of said applications using the Mg-Ti containing complex catalyst possess a narrow molecular weight distribution, Mw/Mn, of about .gtoreq.2.7 to .ltoreq.4.1.
Over the years, film extrusion equipment has been optimized for the rheology of HP-LDPE. The different molecular architecture of LP-LDPE results in a film processing behavior which requires different extrusion parameters. Although LP-LDPE resins can be extruded on equipment designed for HP-LDPE resins, certain equipment modifications are often required in order to extrude the low pressure resins at optimum conditions and at rates comparable to the high pressure resins. This is particularly true during extrusion of LP-LDPE which is processed into film. The problem appears to be that during extrusion of these particular resins, two aspects or rheological behavior play a significant role, i.e. shear and extension. Within a film extruder and extrusion die, a polymeric melt undergoes severe shearing deformation. As the extrusion screw pumps the melt to, and through, the film die, the melt experiences a wide range of shear rates. Most film extrusion processes are thought to expose the melt to shear at rates in the 100-5000 sec.sup.-1 range. Polymeric melts are known to exhibit what is commonly termed shear thinning behavior, i.e., non-Newtonian flow behavior. As shear rate is increased, viscosity (the ratio of shear stress, .tau., to shear rate, .gamma.) decreases. The degree of viscosity decrease depends upon the molecular weight, its distribution, and molecular configuration, i.e., long chain branching of the polymeric material. Short chain branching has little effect on shear viscosity. In general, high pressure low density polyethylenes have a broad molecular weight distribution and show enhanced shear thinning behavior in the shear rate range common to film extrusion. The narrow molecular weight distribution resins used in the present invention exhibit reduced shear thinning behavior at extrusion grade shear rates. The consequences of these differences are that the narrow distribution resins used in the present invention require higher power and develop higher pressures during extrusion than the high pressure low density polyethylene resins of broad molecular weight distribution and of equivalent average molecular weight.
The rheology of polymeric materials is customarily studied in shear deformation. In simple shear the velocity gradient of the deforming resin is perpendicular to the flow direction. The mode of deformation is experimentally convenient but does not convey the essential information for understanding material response in film fabrication processes. As one can define a shear viscosity in terms of shear stress and shear rate, i.e.: EQU .eta.shear=.tau..sub.12 /.gamma.
where
.eta.shear=shear viscosity (poise)
.tau..sub.12 =shear stress (dynes/cm.sup.2)
.gamma.=shear rate (sec.sup.-1)
an extensional viscosity can be defined in terms of normal stress and strain rate, i.e.,: EQU .eta.ext=.pi./.epsilon.
.rho.ext=extensional viscosity (poise)
.pi.=normal stress (dynes/cm.sup.2)
.epsilon.=strain rate (sec.sup.-1)
Due to the high shear stress developed during extrusion of a high molecular weight ethylene polymer having a narrow molecular weight distribution, melt fracture, particularly sharkskin melt fracture, occurs. Sharkskin melt fracture has been described in the literature for a number of polymers. "Sharkskin" is a term used to describe a particular type of surface irregularity which occurs during extrusion of some thermoplastic materials under certain conditions. It is characterized by a series of ridges perpendicular to the flow direction and is described by J. A. Brydson, Flow Properties of Polymer Melts, Van Nostrand-Reinhold Company (1970), pages 78-81.
In the present process, the onset of sharkskin melt fracture is determined by visual observation of the surface of an extrudate formed without take-off tension from a capillary die. Specifically, this procedure for determining sharkskin melt fracture is as follows: The extrudate is lighted from the side and examined under a 40X magnification microscope. The microscope shows the transition from a low-shear, glossy surface of the extrudate to a critical-shear, matted surface (the onset of sharkskin melt fracture) to a high-shear, deep ridge, sharkskin melt fracture. This method is generally reproducible to .+-.10 percent in shear stress.
The narrow molecular weight distribution ethylene polymers as described herein exhibit the characteristics of sharkskin melt fracture upon extruding using the prior art extrusion processes. These characteristics include a pattern of wave distortion perpendicular to the flow direction; occurrence at low extrusion rates (less than expected for elastic turbulance); not effected by the use of commonly employed metal die materials; and less melt fracture with increasing temperature.
There are several known methods for eliminating sharkskin melt fracture in polymers. These methods include increasing the resin temperature. However, in film formation this method is not commercially useful since increasing resin temperature generally causes lower rates of film formation, due to bubble instability or heat transfer limitations. Another method for eliminating sharkskin is described in U.S. Pat. No. 3,920,782. In this method sharkskin formed during extrusion of polymeric materials is controlled or eliminated by cooling an outer layer of the material to close to the fusion temperature so that it emerges from the die with a reduced temperature while maintaining the bulk of the melt at the optimum working temperature. However, this method is difficult to employ and control.
The invention of U.S. Pat. No. 3,920,782 is apparently based on the inventor's conclusions that the onset of sharkskin melt fracture under his operating conditions with his resins is a function, basically, of exceeding a critical linear velocity with his resins through his dies at his operating temperatures. In the process of the present invention, however, the onset of sharkskin melt fracture in the present applicants' resins under their operating conditions is a function, primarily, of exceeding a critical shear stress, and, to a lesser extent, a function of exceeding a critical linear velocity.
More recent attempts have been made to reduce sharkskin melt fracture during extrusion of the particular ethylene polymers disclosed herein by geometric changes in the die. For example Application Ser. No. 099,061 filed on Dec. 12, 1979 now U.S. Pat. No. 4,282,177 issued Aug. 4, 1981 and which is a continuation-in-part of Application Ser. No. 001,932 filed Jan. 8, 1979, now U.S. Pat. No. 4,267,146 issued May 12, 1981 discloses a method for reducing sharkskin melt fracture during extrusion of a molten narrow molecular weight distribution linear ethylene polymer by extruding the polymer through a die having a die gap greater than about 50 mils and wherein at least a portion of one surface of the die lip and/or die land in contact with the molten polymer is at an angle of divergence or convergence relative to the axis of flow of the molten polymer through the die. In addition, Application Ser. No. 012,793 filed on Feb. 16, 1979 now U.S. Pat. No. 4,271,092, discloses a process for forming blown tubular film essentially free of melt fracture by extruding the particular polymer through an extrusion die having a die gap of greater than about 50 mils and at a drawdown ratio of greater than about 2 to less than about 250.
In the process of the present invention melt fracture, particularly sharkskin melt fracture, can be virtually eliminated on one surface of an extruded film formed from the polymers contemplated herein, by geometric changes in the die, i.e., by extruding the narrow molecular weight distribution ethylene polymer, at normal film extrusion temperatures through a die having a discharge outlet defining an exit die gap and wherein one surface of the die lip and/or die land in contact with the molten polymer extends beyond the opposing surface of the die lip and/or die land in the direction of the axis of flow of the molten polymer through the die land whereby melt fracture is reduced on the surface of the polymer leaving the extended die lip surface. The utility of the process of the present invention arises due to the fact that the stress field at the exit of the die determines the creation of sharkskin melt fracture. Thus, sharkskin melt fracture can be controlled or eliminated by the geometry at the exit of the die and is independent of die land conditions.
Films suitable for packaging applications must possess a balance of key properties for broad end-use utility and wide commercial acceptance. These properties include film optical quality, for example, haze, gloss, and see-through characteristics. Mechanical strength properties such as puncture resistance, tensile strength, impact strength, stiffness, and tear resistance are important. Vapor transmission and gas permeability characteristics are important considerations in perishable goods packaging. Performance in film converting and packaging equipment is influenced by film properties such as coefficient of friction, blocking, heat sealability and flex resistance. Low density polyethylene has a wide range of utility such as in food packaging and non-food packaging applications. Bags commonly produced from low density polyethylene include shipping sacks, textile bags, laundry and dry cleaning bags and trash bags. Low density polyethylene film can be used as drum liners for a number of liquid and solid chemicals and as protective wrap inside wooden crates. Low density polyethylene film can be used in a variety of agricultural and horticultural applications such as protecting plants and crops, as mulching, for storing of fruits and vegetables. Additionally, low density polyethylene film can be used in building applications such as a moisture or moisture vapor barrier. Further, low density polyethylene film can be coated and printed for use in newspapers, books, etc.
Possessing a unique combination of the aforedescribed properties, high pressure low density polyethylene is the most important of the thermoplastic packaging films. It accounts for about 50% of the total usage of such films in packaging. Films made from the polymers of the present invention, preferably the ethylene hydrocarbon copolymers, offer an improved combination of end-use properties and are especially suited for many of the applications already served by high pressure low density polyethylene.
An improvement in any one of the properties of a film such as elimination or reduction of sharkskin melt fracture or an improvement in the extrusion characteristics of the resin or an improvement in the film extrusion process itself is of the utmost importance regarding the acceptance of the film as a substitute for high pressure low density polyethylene in many end use applications.
In the case where a single layer film is extruded consisting entirely of LP-LDPE resin, the reduction in melt fracture would occur on the surface of the film in contact with the extended surface of the die. For this reason, the process of the present invention is particularly suitable for the formation of multilayer films wherein one layer is formed of LP-LDPE and another layer is formed from a resin which under the conditions of operation is not subject to melt fracture. Thus, by the process of the instant invention, the LP-LDPE resin can be passed through the die in contact with the extended die lip surface whereas the resin not subject to melt fracture is extruded in contact with the shorter exit die lip surface thereby producing a multi-layer film, both outer surfaces of which would be free of melt fracture.